Prior single channel adaptive line enhancers that employ up/down conversion require that unrealistic constraints be imposed on the internal bandpass filter in order to achieve satisfactory performance. For instance, in order to avoid excessive folding in at D.C., bandpass filter gain should be down at least 30 dB at the local oscillator frequency. However, filter gain should be near 0 dB throughout the rest of the passband in order to achieve reasonable narrowband rejection figures.
This is true because in order to achieve the best performance from the adaptive line enhancer, the weight accumulators of the line enhancer should utilize the full dynamic range available to them. But attenuation in the narrowband channel causes the values in the weight accumulators to increase in order to compensate for this attenuation, which eventually results in railing of the weight accumulators with a consequent degradation of filter performance. If a passband of 50 MHz which is centered at 1.7 GHz is required and both of the above constraints are imposed, bandpass filters become impractical. In order to compensate, two stage down conversion may be attempted, but the resulting greater complexity, larger hardware requirements and an increased residual error because of phase noise make this approach undesirable.
Adaptive line enhancers (ALE) employ feedback to enhance the narrowband frequency spectrum that is present in a broadband noise field or to suppress narrowband interference in a broadband signal. Adaptive line enhancers are constructed so that they automatically adjust to variations in the input signal in order to provide a least mean square (LMS) approximation of a Wiener-Hopf filter. This device uses a number of stored weight values which are continuously adjusted so that the device automatically filters out the components of the signal which are not correlated in time, and passes correlated stable spectral lines. This enhanced narrowband signal is subtracted from the composite input signal to generate the error or feedback signal.
The McCool et al U.S. Pat. No. 4,238,746 issued Dec. 9, 1980 entitled "Adaptive Line Enhancer," the McCool et al U.S. Pat. No. 4,243,935 issued Jan. 6, 1981 entitled "Adaptive Detector" and the article entitled "The Time-Sequenced Adaptive Filter" by Ferrara, Jr. and Widrow which was published 1981 in the IEEE Transactions on Circuits and Systems, Vol. CAS-28 (1981) June N 6, New York USA show prior art devices and/or provide discussions of the theoretical basis of the adaptive line enhancer.